This invention relates to a composition comprising antimony trifluoride and silica, a method for the preparation of said composition and certain uses of said composition, in particular its use as catalyst.
The Baeyer-Villiger oxidation of ketones to lactones or esters (A. Baeyer and V. Villiger, Ber., 1899, vol. 32, page 3625) is a widely used synthetic method and has been comprehensively reviewed (C. H. Hassall, Org. React., 1957, vol. 9, page 73; G. R. Krow, Org. React., 1993, vol. 42, page 251). An example is the oxidation of cyclohexanone to xcex5-caprolactone, which is of particular industrial interest since it is extensively used in the synthesis of polycaprolactones.
The most common reagents used for the Baeyer-Villiger oxidation of ketones to lactones or esters are peroxycarboxylic acids (e.g. peracetic acid, m-chloroperbenzoic acid). These are commercially available on a large scale, but their industrial use is impractical for safety and cost reasons. In particular, they are shock sensitive in their pure form and may be explosive in the condensed phase. Moreover, they yield the corresponding carboxylic acid products, which may give rise to separation and recycling problems.
In order to avoid the use of these potentially dangerous materials and to lower the environmental impact of the waste produced, various other oxidation systems using hydrogen peroxide as the oxidant have been developed. (J. D. McClure et al., J. Org. Chem., 1962, vol 27,24; S E. Jacobsen et al., J. Am. Chem. Soc., 1979, vol. 101,6938; A. Gusso et al., Organometallics, 1994, vol. 13, 3442). For example, the following compounds have been used in conjunction with hydrogen peroxidexe2x80x94boron trifluoride etherate, borax (Na2B4O7), arsenic containing compounds, cationic platinum complexes, molybdenum complexes, methyltrioxorhenium, selenium compounds, titanium silicate, zeolites, sulfonated resins and sodium hydroxide.
Tanabe and Hattori have made solid superacids based on antimony pentafluoride supported on silica and other metal oxides (K. Tanabe et al., Chem. Lett., 1976, 625), but antimony trifluoride supported on silica has not been reported or even suggested, and neither has its use as catalyst in Baeyer-Villiger oxidations.
Accordingly, the present invention provides a composition comprising antimony trifluoride and silica. As used herein, xe2x80x9csilicaxe2x80x9d is defined as any form of a dioxide of silicon. Forms of silica may be hydrated. Forms of silica include a crystalline quartz form; a crystalline cristobalite form; a crystalline tridymite form; a crytocrystalline chalcedony form; an amorphous form; an amorphous opal form; an amorphous granular hydrated form (silica-gel); a form including aluminium atoms, such as an aluminosilicate; and a mesoporous form, such as HMS hexagonal mesoporous silica. Preferably the silica comprises HMS or silica-gel; more preferably the silica comprises HMS.
Preferably the composition is substantially pure. If present, materials other than antimony trifluoride and silica are preferably present in only trace amounts. Preferably the antimony trifluoride is present in an amount of up to about 2 mmol SbF3 per gram of composition, more preferably the antimony trifluoride is present in an amount of between 0.1 and 1.5 mmol, preferably 0.5 to 1.5 mmol SbF3 per gram of composition. Amounts of 1 to 2 mmol SbF3 per gram of composition can lead to good results.
Preferably, the composition is in the form of a powder having a mean particle size, specific surface area (SA) and average pore diameter (APD) which depend on those of the starting silica. Generally, the SA is below or equal to 1300 m2/g, in particular 1250 m2/g, or even below or equal to 1000 m2/g in the case of HMS, and below or equal to 600 m2g, in particular 530 m2/g, or even below or equal to 450 m2/g in the case of silica-gel. The SA is generally above 300 m2/g, and even above 700 m2/g in the case of HMS. And typically, the APD is below or equal to 10 nm, in particular 6 nm. The APD is generally above 1 nm, in particular 2 nm. The APD is mostly close to or superior to 3 nm in the case of HMS, and it may be close to 6 nm in the case of silica-gel.
The present invention further comprises a process comprising the use of the composition comprising antimony trifluoride and silica as a catalyst. Reactions may be carried out as a batch process or in a continuous process. Preferably the composition catalyses an organic oxidation reaction. Preferably hydrogen peroxide is used as oxidising agent. Preferably the composition calalyses an oxidation of a ketone to a lactone or ester, more preferably the composition catalyses the oxidation of cyclohexanone to xcex5-caprolactone.
It may also catalyse the oxidation of cyclopentanone to valerolactone and the oxidation of cycloheptanone to enantholactone.
The oxidation of cyclohexanone to xcex5-caprolactone may be carried out as a batch process or a continuous process. In a batch process, the composition, cyclohexanone and hydrogen peroxide are mixed and allowed to react. In a continuous process, cyclohexanone and hydrogen peroxide may, for example, be passed through a column containing the composition. The product may be removed from the reaction mixture in subsequent separation steps.
Preferably the process of the oxidation of cyclohexanone to xcex5-caprolactone comprises the steps of:
a) mixing the composition with cyclohexanone,
b) adding hydrogen peroxide to the mixture, and
c) allowing the reaction to proceed.
These steps can be carried out successively, in a batch process, or simultaneously in a continuous process.
Typically, step (a) is performed at a temperature of up to about 70xc2x0 C. More preferably, step (a) is performed at room temperature. Typically step (b) is performed over a period of between about 5 minutes to about 120 minutes preferably over about 30 minutes. Typically in steps (b) and (c) the reaction is maintained at a temperature of up to about 110xc2x0 C., preferably at a temperature of between about 60xc2x0 C. and about 100xc2x0 C., more preferably between about 70xc2x0 C. and 90xc2x0 C. It has been found that raising the temperature can improve yields but is detrimental to the recovery of the catalyst. Typically step (c) is performed at a pressure at which the reaction mixture boils (in order to remove water azeotropically). Preferably step (c) is performed at a pressure of between 50 and 150 mbar. Preferably steps (a), (b) and (c) is performed under stirring.
Preferably a molar excess of cyclohexanone is used relative to hydrogen peroxide. Preferably the molar ratio of cyclohexanone: hydrogen peroxide is up to about 5:1, more preferably from about 2:1 to about 3:1, more preferably about 2.5:1. Preferably the composition is used in an amount of up to 10 g per 100 ml of cylcohexanone, more preferably the composition is used in an amount of up to 5 g per 100 ml of cyclohexanone, even more preferably the composition is used in an amount of up to 2 g per 100 ml of cyclohexnone.
Preferably water is constantly removed from the reaction mixture. This gives the advantage of limiting the formation of unwanted by-products which can be formed through the ring opening of the lactone under the action of water. Any suitable means may be used for removing water, for example azeotropic removal or use of a dessicant. Preferably the water is removed azeotropically.
Preferably hydrogen peroxide is a 30% to 90% solution in water, more preferably hydrogen peroxide is a 70 to 85% solution in water, for instance an about 85% solution in water. However, a 65-75%, and especially a 70% H2O2 solution was surprisingly more effective than a 80-90%, and especially a 86% H2O2 solution, which shows that the presence of a small amount of water during the reaction may be beneficial.
Alternative oxidising agents include agents capable of generating hydrogen peroxide in situ and other peroxides such as sodium percarbonate and sodium perborate.
The present invention further provides a method for the preparation of the composition comprising antimony trifluoride and silica, comprising the step of treating silica with antimony trifluoride. Preferably the silica is selected from the group consisting of HMS and silica-gel.
Preferably the method comprises the steps of:
a) synthesising HMS using a template,
b) removing the template either by calcination to yield a calcinate HMS support (herein exemplified as HMS-C) or by solvent extraction and subsequent drying to yield a dried HMS support (herein exemplified as HMS-E), and
c) treating the calcinated HMS with antimony trifluoride to yield a composition of the present invention or theating the dried HMS with antimony trifluoride to yield a composition of the present invention.
Alternatively the method comprises the steps of:
a) calcinating silica-gel to yield a calcinated silica-gel support (herein exemplified as K-600), or drying silica-gel to yield a dried silica-gel support (herein exemplified as K-120), and
b) treating the calcinated silica-gel with antimony trifluoride to yield a composition of the present invention or treating the dried silica-gel with antimony trifluoride to yield a composition of the present invention.
Preferably the calcination of the silica is performed at a temperature in the rang of about 250xc2x0 C. to about 650xc2x0 C. Preferably the drying of the silica is performed at a temperature in the range of about 100xc2x0 C. to about 250xc2x0 C.
Preferably the step of treating the silica with antimony trifluoride comprises the step of treating the silica with a refluxing mixture of antimony trifluoride in toluene. The handling (exposure to air) of SbF3 should be avoided prior to this treatment step.